This invention relates generally to the art of making and using oilfield treatment fluids viscosified with surfactants that gel in aqueous systems. More particularly it relates to perforating fluids and methods of perforating in a well from which oil and/or gas can be produced. Most particularly it relates to perforating fluids and methods of perforating applying to a well with casing and/or having high density brines.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In order to produce hydrocarbon fluids from subterranean formations, a borehole is drilled from the surface down into the desired formations. Typically, casing is placed in the borehole, thereby defining a hollow wellbore. In order for the hydrocarbon fluids to flow from the surrounding formations into the wellbore and up to the surface, it is necessary to perforate the casing. This is typically done using a perforating gun, a downhole tool that detonates explosive charges at selected locations in order to form holes in the casing.
Because the fluids in the formation are under pressure, a choice must be made whether to perforate the well with the bottom-hole pressure in the wellbore lower or higher than the formation pressure. The former condition is referred to as “underbalanced” and the latter condition is referred to as “overbalanced”.
It has been common to perforate the casing of a well in an underbalanced condition to minimized impairment to the formation permeability. Placing a perforating fluid between the formation and the hollow wellbore that prevents leakoff in an overbalanced condition allows the well to be perforated but restricted from producing and therefore under control. An operator can then safely complete the perforating and subsequent well completion operations without loss of fluids into the formation and without movement of fluids from the formation into the wellbore. This perforating fluid now allows high pressure reservoirs where an underbalance condition is easy to achieve as well as low pressure reservoirs where it is not possible to achieve an under-balanced condition can be perforated overbalanced.
Experience has shown that different wellbore fluids can have significant effects on production rates. For example, perforating with brine in the wellbore usually results in two-phase (oil and water) flow in the formation pores. This condition is known to deleteriously affect production rates. It is also possible to perforate with an oil-based mud in the wellbore. This eliminates the two-phase flow in the formation pores, but can results in plugging some pores with the solid (clay) portion of the mud.
Viscous fluids play an important role in oilfield service applications ranging from fracturing fluids, gravel pack fluids, drilling fluids, cleanout fluids, acid diverting reagents, fracture fluid diverting agents and many more. Nearly all of the traditional fluids for these applications are polymer based and have several intrinsic drawbacks due to their polymeric nature, such as irreversible formation damage and high friction pressure. A new technology based on viscoelastic surfactant (VES) fluids has several distinctive advantages over polymer-based fluids. VES based fluids are excellent particle suspension media; which form entangled micelle structures. Unlike polymer fluids, when the VES systems break they form solids-free fluids, minimizing damage wherever they are used. However, many VES fluids are very sensitive towards high brine concentrations, especially heavy brines, often not gelling at all. Therefore their use as fluids for gravel pack applications, drill-in or completion fluids, especially for deep wells, and all other applications demanding heavy fluids to balance the well pressure is still very limited. In order to overcome these limitations, new developments of brine insensitive fluids are necessary.
The fluids of embodiments of the present invention are the VES fluids that can best meet the density and stability requirements. VES fluid systems are advantageously used for coiled tubing cleanout applications, in particular in horizontal or inclined wellbores, because VES fluids have superior drag reduction properties (that is, low friction pressures); they can be foamed; they generally undergo micelle disruptive shear thinning at high shear rates (high flow rates and/or great turbulence in the tubing); the very low viscosity fluid that has undergone disruptive shear thinning can enter the sand mass and pick the sand up easily; and since they recover viscosity quickly, the sand will remain suspended as it is moved back up the wellbore.
Further, such fluids have been generally used as completion fluids, fracturing fluids and the like, while conventional materials such as mineral acids, organic acids and chelation agents have been employed as perforating fluids. Such fluids however, when used alone, may introduce damage into the formation and/or be subject to dilution or precipitate due to incompatibility with to the high-density brines. Further, such fluids when used alone may not be capable of effectively removing perforating debris from the wellbore.
There is a need for improved methods of perforating well that reduce or eliminate at least some of the above-described problems.
It has now been discovered that a certain viscoelastic surfactants can be used as a perforating fluid to stop fluid leak-off, even with high density brines.
It has now been discovered that certain viscoelastic surfactants can be used to stop fluid leak-off in scenarios where the well has been perforated prior to the introduction of the viscoelastic surfactants.